1. Technical Field
The present invention concerns a method and a apparatus for feeding electric current into an electric three-phase network. The present invention also concerns a wind power installation which is adapted to feed electric current into a three-phase network.
2. Description of the Related Art
Methods and apparatuses for feeding electric current into an electric three-phase network such as for example the European integrated network or into a part thereof are generally known. Large-scale power stations use for that purpose a synchronous generator connected directly to the electric network. For that purpose the synchronous generator is operating at a suitable rotary speed which is precisely matched to the frequency of the electric network. Depending on the respective structural configuration of the synchronous generator the rotary speed is for example 1500 revolutions per minute in the case of a four-pole synchronous generator connected to a network involving a frequency of 50 Hz. If disturbances occur in the network such as for example an asymmetric loading on the network in which for example one of the three network phases is more heavily loaded, that has a direct effect on the current delivered by the synchronous generator. In that case the physically governed characteristics of the synchronous generator can at least partially contribute to making the network symmetrical again. The nature and fashion of such a contribution from the synchronous generator however cannot basically be influenced because of the rigid coupling to the network.
Wind power installations were still connected passively to the network in the 1990's in the sense that they feed as much energy into the network as is possible at the respective current moment in time in consideration of the prevailing wind conditions. It was recognized for the first time at the end of the 1990's that wind power installations can also make a contribution to electrically supporting the network. Thus for example German patent application DE 100 22 974 A1 describes a method in which wind power installations can change and in particular throttle their feed of power into the network in dependence on the network frequency. DE 101 19 624 A1 proposes that in the event of a network disturbance, more specifically in particular in the case of a short-circuit, a wind power installation restricts the current which it feeds into the network instead of being disconnected from the network in order also thereby to achieve network support. WO 02/086315 A1 describes a method for network support by a wind power installation, which adjusts a phase angle of the fed-in current in dependence on the network voltage and thus feeds apparent power into the network in dependence on voltage in order thereby to support the network. DE 197 56 777 A1 also concerns a method of network support by means of a wind power installation, in which the wind power installation possibly reduces the power to be fed into the network, in dependence on the network voltage, in order thereby in particular to avoid disconnection from the network, in order also thereby to achieve support for the network by a wind power installation.
Wind power installations are becoming increasingly important. They are also increasingly gaining in importance in regard to network support. The above-described network support measures—which can also be referred to as pioneering steps—are however still capable in that respect of being improved in so far as network asymmetries are taken into account.
In that respect taking account of network asymmetries raises some problems. Firstly problems are involved in rapidly and accurately detecting network asymmetries. In addition in the case of detection of asymmetries, the problem arises of targetedly compensating for same, which is not possible or is only limitedly possible with a strongly coupled synchronous generator. The same problems occur in systems which do not use a synchronous generator but simulate same in terms of behavior.
As state of the art attention is also to be directed generally to WO 2010/028689 A1 relating to a wind power installation with a double-fed asynchronous machine.